The present invention relates to foarmable microspheres with a foaming agent enclosed in the shell of a polymer, and more particularly to foamable microspheres extremely sharp in particle diameter distribution. The present invention also relates to a process for producing foamable microspheres extremely sharp in particle diameter distribution by a suspension polymerization process. The present invention further relates to a process for producing foamable microspheres, by which aggregation of polymer particles formed and adhesion of scale to the wall of a polymerization vessel upon polymerization are prevented, and foamable microspheres even in particle shape in the form of a sphere and capable of sharply foaming to provide uniform foams can be provided. The foamable microspheres according to the present invention can be utilized in a wide variety of technical fields including fields of paint and ink.
In recent years, foamable microspheres have been developed into uses in various fields such as fillers for paints and plastics for the purpose of lightening weight. including the use of foaming ink. The foamable microspheres are generally obtained by microcapsulating a volatile, liquid foaming agent (also referred to as a physical foaming agent or volatile expanding agent) by a thermoplastic resin. Such foamable microspheres have heretofore been produced by a process in which a polymerizable mixture containing at least a foaming agent and a polymerizable monomer is subjected to suspension polymerization in an aqueous dispersion medium. With the progress of the polymerization reaction, a shell is formed by a polymer formed, thereby obtaining the foamable microspheres containing the foaming agent encapsulated in the shell.
In the suspension polymerization process, the polymerizable mixture is generally added to the aqueous dispersion medium containing a dispersion stabilizer, the aqueous dispersion medium containing such a mixture is stirred and mixed to form fine droplets of the polymerizable mixture, and the resultant dispersion is then heated to conduct suspension polymerization. Since the polymerizable mixture forms an oil phase in the aqueous dispersion medium, it can be formed into fine droplets by stirring and mixing. By the suspension polymerization, foamable microspheres having substantially the same particle diameter as these fine droplets are formed. In the step of forming the fine particles of the polymerizable mixture, the stirring and mixing have heretofore been conducted by means of a general agitating blade or a batch-wise high-speed, high-shear type dispersing machine. In the suspension polymerization process, foamable microspheres, the particle shape of which is made even in the form of a sphere, can be provided by suitably selecting a dispersion stabilizer, a polymerization aid and the like. Accordingly, foamable microspheres having properties satisfactory to some extent can be obtained by devising suspension polymerization conditions even when such stirring and mixing method as described above is adopted.
However, when application fields of foamable microspheres are enlarged, and higher performance comes to be required in each application field, the level required of the foamable microspheres is also raised. As the performance of the foamable microspheres, it is particularly important that foaming is sharp, and foams uniform in shape and size can be formed. The phrase xe2x80x9cforming is sharpxe2x80x9d as used herein means that foam initiating temperatures of individual particles of the foamable microspheres are substantially the same, and the particles initiate foaming all at once under foaming temperature conditions. Therefore, the foamable microspheres are required to have an extremely narrow particle diameter distribution in addition to the even particle shape in the form of a sphere. However, the foamable microspheres obtained in accordance with the conventional process is not sufficiently sharp in particle diameter distribution and hence contain minute particles and coarse particles in plenty based on an average particle diameter. When the particle diameter distribution of the foamable microspheres is broad as described above, foaming conditions among individual particles are delicately different, and so foaming cannot be sharply conducted. In addition, when the particle diameter distribution is broad, foams of uniform size cannot be obtained. Such a tendency is particularly marked when the average particle diameter of the foamable microspheres is great. On the other hand, when classification is conducted to narrow the particle diameter distribution of the foamable microspheres, the process becomes complicated, and a yield is lowered.
The above-described problems are described by specific examples. For example, the use of foamable microspheres broad in particle diameter distribution as a weight-lightening agent or functionality-imparting agent for high-performance paints arises a problem that a finished surface becomes rough due to the presence of coarse particles. The coarse particles are easy to foam at a low temperature to impair sharp foaming. The coarse particles also involves that an expansion ratio cannot be raised because the foaming agent is easy to escape. The minute particles involves a problem that an expansion ratio cannot be raised because the content of the foaming agent therein is low. Such problems become a fatal defect in use of the foamable microspheres for an extremely thin coating film capable of making the best use of the feature thereof.
The foamable microspheres not only are incorporated into ink, paint, plastics and the like in an unformed state, but also may be used in a foamed state according to their uses. More specifically, foams (hollow plastic balloons) of the foamable microspheres are very light-weight and hence come to be used as a filler for paints, for example, for the purpose of lightening the weight of an object to be coated, such as an automobile. Since the foams are very light fine particles generally having a bulk density of about 0.02 to 0.03 g/cm3 and an average particle diameter of about 20 to 200 xcexcm, they are easy to escape out in the air when they are taken out of a container and incorporated into a base material for a paint or the like. In addition, the foams gather on the top of the base material upon their stirring and mixing with the base material, and so it is very difficult to uniformly mix them.
Therefore, Japanese Patent Application Laid-Open No. 196813/1995 has proposed a production process of non-scattering foamed microspheres (non-scattering hollow plastic balloons) in which unformed foamable microspheres are mixed with a plasticizer at a temperature lower than the foam initiating temperature of the foamable microspheres, the resultant mixture is brought into contact with another plasticizer heated to a temperature higher than the foam initiating temperature of the foamable microspheres to foam the formable microspheres, and the foamed microspheres are cooled to prevent overfoaming. According to this process, there are merits that {circle around (1)} the unformed foamable microspheres can be dispersed in the plasticizer to make them a fluid state permitting quantitative feeding by a pump, {circle around (2)} uniform non-scattering foamed microspheres can be obtained at the same time as the foaming of the foamable microspheres, and {circle around (3)} the amount of the plasticizer used can be very lessened compared with a process of simply wetting with a plasticizer. In order to adopt such a process, the foamable microspheres are required to have the following properties:
(1) having good solvent resistance because a plasticizer is used as a wetting agent;
(2) exhibiting a viscosity as low as possible when the foamable microspheres are dispersed in the plasticizer in order that the microspheres can be quantitatively fed by a pump;
(3) causing sharp foaming from the viewpoints of process and product quality; and
(4) forming no aggregate upon a foaming process.
Accordingly, there is a demand for development of foamable microspheres having these properties (1) to (4) in combination. However, the conventional foamable microspheres are broad in particle diameter distribution and hence cannot fully satisfy these required properties.
On the other hand, a production process of foamable microspheres by the suspension polymerization process tends to cause problems that polymer particles formed aggregate and scale adheres to the wall of a polymerization vessel upon polymerization. Therefore, various processes for producing foamable microspheres by devising a polymerization aid and a dispersion stabilizer have heretofore been proposed. However, the conventional production processes have involved various problems and been not fully satisfactory.
For example, Japanese Patent Publication No. 26524/1967 describes unicellular particles (i.e., foamable microspheres) of a thermoplastic resinous polymer with a volatile liquid foaming agent, which becomes a gaseous state at a temperature lower than the softening point of the polymer, enclosed therein. This publication discloses a process for producing spherical particles with the foaming agent enclosed in a shell formed of the thermoplastic resin by adding the foaming agent such as a low-boiling aliphatic hydrocarbon to a monomer, mixing an oil-soluble catalyst with the resultant monomer mixture and then adding the monomer mixture to an aqueous dispersion medium containing a dispersing agent with stirring to conduct suspension polymerization. Japanese Patent Application Laid-Open No. 286534/1987 describes a process for producing heat-expanding microcapsules (i.e., foamable microspheres) by using a polymer obtained from a component comprising at least 80 wt. % of a nitrile monomer, at most 20 wt. % of a non-nitrile monomer and a crosslinking agent to enmicrocapsulating a volatile expanding agent. In these conventional production processes, a polymerizable mixture comprising a foaming agent, a polymerizable monomer and a polymerization initiator is subjected to suspension polymerization in an aqueous dispersion medium containing colloidal silica as a dispersion stabilizer (suspending agent), a diethanolamine-adipic acid condensation product as an auxiliary stabilizer and potassium bichromate as a polymerization aid, thereby producing the foamable microspheres.
However, potassium bichromate used as the polymerization aid in these conventional processes involves a problem that it has toxicity. In addition, when potassium bichromate is used, the resulting foamable microspheres are colored yellow due to a remaining chromium ion, thereby impairing the color tone of various products comprising such microspheres in an unformed or foamed state. When such yellow microspheres are caused to be contained in a colored product in particular, the color tone of the product tends to become dull.
When potassium bichromate is not used upon the suspension polymerization, polymer particles formed show a tendency to aggregate, or a problem that a polymer formed adheres as scale to the wall of a polymerization vessel is easy to arise. When the polymer particles aggregate, the viscosity of the suspension polymerization reaction system is increased to adversely affect the progress of the polymerization reaction and the particle shape of the resulting foamable microspheres. When polymer scale covers the wall of the polymerization vessel, the heat removing ability of the polymerization vessel is lowered, and a yield of the foamable microspheres is reduced. When aggregates of the polymer particles or peeled matter from polymer scale adhered are mixed in the foamable microspheres, a problem that a finished surface becomes rough arises when such foamable microspheres are used as, for example, a weight-lightening agent or functionality-imparting agent for high-performance paints, since these aggregates and peeled matter from polymer scale are coarse particles. These coarse particles and particles in a form out of sphere cause problems that they are easy to foam at a low temperature compared with spherical particles, and that the foaming agent is easy to escape, and so an expansion ratio cannot be raised. Such problems become a fatal defect in use of the foamable microspheres for, in particular, an extremely thin coating film capable of making the best use of the feature thereof. In the case of their use for air spray, the clogging of a gun and uneven coating tend to occur.
Japanese Patent Application Laid-Open No. 292643/1992 (Patent No. 2584376) discloses a process for producing foamable thermoplastic microspheres by using, as a suspending agent (dispersion stabilizer), a powdered stabilizer insoluble in an aqueous medium at a pH that the aqueous medium has upon polymerization, such as magnesium hydroxide. This publication states that xe2x80x9cAccording to this process, the powdered stabilizer can be dissolved and removed by lowering the pH of the aqueous medium after the polymerization, so that foamable microspheres having a clean polymer surface can be provided.xe2x80x9d However, the problem that polymer particles aggregate cannot be solved even by this process.
It is an object of the present invention to provide foamable microspheres which have a spherical particle shape, are extremely sharp in particle diameter distribution and are capable of sharply foaming to provide uniform foams.
Another object of the present invention is to provide a process for producing such foamable microspheres sharp in particle diameter distribution.
A further object of the present invention is to provide a process for producing foamable microspheres, by which aggregation of polymer particles formed and adhesion of scale to the wall of a polymerization vessel upon polymerization are prevented, and foamable microspheres even in particle shape in the form of a sphere and capable of sharply foaming to provide uniform foams can be provided.
The present inventors have carried out an extensive investigation with a view toward overcoming the above-described problems involved in the prior art. As a result, it has been found that in a process for producing foamable microspheres with a foaming agent enclosed in the shell of a polymer formed by subjecting a polymerizable mixture containing at least the foaming agent and a polymerizable monomer to suspension polymerization in an aqueous dispersion medium, the aqueous dispersion medium and the polymerizable mixture are fed to a continuous high-speed, high-shear type stirring and dispersing machine, continuously stirring both in the stirring and dispersing machine so as to disperse the polymerizable mixture in the aqueous dispersion medium, and the resultant dispersion is then poured into a polymerization tank to conduct suspension polymerization in the polymerization tank, whereby foamable microspheres having an average particle diameter within a range of 3 to 100 xcexcm and an extremely sharp particle diameter distribution of at most 1.50% in terms of the coefficient of variation of the particle diameter distribution are provided. The foamable microspheres are novel and can sharply foam due to their low contents of coarse particles and minute particles to provide uniform foams.
As a result of the extensive investigation, the present inventors have also found that in a process for producing foamable microspheres with a foaming agent enclosed in the shell of a polymer formed by subjecting a polymerizable mixture containing at least the foaming agent and a polymerizable monomer to suspension polymerization in an aqueous dispersion medium, the suspension polymerization of the polymerizable mixture is conducted in the presence of at least one compound selected form the group consisting of alkali metal nitrites, stannous chloride, stannic chloride, water-soluble ascorbic acids and boric acid, whereby foamable microspheres can be produced stably without causing aggregation of polymer particles formed upon the polymerization while preventing the polymer formed from adhering to the wall of a polymerization vessel and efficiently removing heat generated by the polymerization. The foamable microspheres obtained according to this production process can sharply foam to provide uniform foams due to their low contents of aspherical particles and aggregated particles.
The present invention has been led to completion on the basis of these findings.
According to the present invention, there are thus provided foamable microspheres with a foaming agent enclosed in the shell of a polymer, wherein the average particle diameter of the microspheres is within a range of 3 to 100 xcexcm, and the coefficient of variation of the particle diameter distribution thereof is at most 1.50%.
According to the present invention, there is also provided a process for producing foamable microspheres with a foaming agent enclosed in the shell of a polymer formed by subjecting a polymerizable mixture containing at least the foaming agent and a polymerizable monomer to suspension polymerization in an aqueous dispersion medium, the process comprising feeding the aqueous dispersion medium and the polymerizable mixture into a continuous high-speed, high-shear type stirring and dispersing machine, continuously stirring both in the stirring and dispersing machine so as to disperse the polymerizable mixture in the aqueous dispersion medium, and then pouring the resultant dispersion into a polymerization tank to conduct suspension polymerization in the polymerization tank.
In the step of feeding the aqueous dispersion medium and the polymerizable mixture into the continuous high-speed, high-shear type stirring and dispersing machine, the aqueous dispersion medium and the polymerizable mixture may preferably be continuously fed as separate streams at a fixed ratio into the continuous high-speed, high-shear type stirring and dispersing machine. As another method, may be mentioned a method in which the aqueous dispersion medium and the polymerizable mixture are poured into a dispersing tank, both are stirred in the dispersing tank to primarily disperse the polymerizable mixture in the aqueous dispersion medium, and the resultant primary dispersion is then fed into the continuous high-speed, high-shear type stirring and dispersing machine.
According to the present invention, there is further provided a process for producing foamable microspheres with a foaming agent enclosed in the shell of a polymer formed by subjecting a polymerizable mixture containing at least the foaming agent and a polymerizable monomer to suspension polymerization in an aqueous dispersion medium, the process comprising conducting the suspension polymerization of the polymerizable mixture in the presence of at least one compound selected from the group consisting of alkali metal nitrites, stanous chloride, stannic chloride, water-soluble ascorbic acids and boric acid.